A phenomenon in which the light from a celestial body is temporarily cut off by the presence of another. This may be: (1) the passage of a planetary satellite, such as the Moon, into shadow so that the direct illumination from the Sun normally causing it to shine is cut off; (2) the obscuration of all or part of the Sun by the passage of the Moon directly across it (solar eclipse); (3) the passage of a star belonging to a binary system behind its companion so that the total light received from the system is reduced.
There is some confusion of usage between the terms ╘eclipse' and occultation. If ╘eclipse' is reserved for the cutting off of sunlight by shadow, solar ╘eclipses' and the phenomena observed in eclipsing binary stars are, strictly speaking, occultations. However, the use of ╘eclipse' in these contexts is firmly established and normal.
In the description of the motion of the moons of other planets, such as Jupiter, it is usual to distinguish between eclipses and true occultations.
The Moon's orbit around the Earth is inclined at only 5í to the plane of the Earth's orbit around the Sun. From time to time, the three bodies become aligned and an eclipse of the Sun or Moon occurs (see illustration).
The Moon's shadow on the Earth is only a few hundred kilometres wide. It traces out a curved path as the motion of the three bodies makes the eclipse visible at successive locations. Over a wider region either side of the path of totality, a partial eclipse is seen. Partial eclipses may occur when no part of the Earth witnesses a total eclipse.
Lunar eclipses occur when the Moon passes into the shadow of the Earth. They can take place only close to full Moon and can be seen from any location where the Moon has risen. The Moon does not normally disappear completely: its disc is illuminated by light scattered by the Earth's atmosphere. It often takes on a deep reddish hue. The full shadow (umbra) cast by the Earth is surrounded by a region of partial shadow, called the penumbra. In the early and late stages of the progress of a lunar eclipse, the Moon enters the penumbra. It is possible for lunar eclipses to occur which are only penumbral. The length of the Moon's path through the umbra, divided by the Moon's apparent diameter, defines the ╘magnitude' of a lunar eclipse.
The relative motions of the Sun, Earth and Moon are such that at least two eclipses of the Sun must occur in any year (though most will be partial). The maximum number of eclipses in any one year is seven, two or three of which must be lunar. It is theoretically possible for solar eclipses to occur at successive new Moons, and for there to be a lunar eclipse in between. However, lunar eclipses at two successive full Moons are not possible.
